TRENDS IN SYSTEMATIC BOTANY
93
facultatively apomictic. It is presumed that the plants of higher chromosome numbers in P. incanum reproduce similarly. Introgression is more
prevalent in the polyploid, facultatively apomictic populations than in
those at the 36-chromosome level. It is highly improbable that these two
species evolved in separate areas and have since come together. Rather,
evolution appears to have taken place, including polyploidy and finally
apomicty, under conditions of continuous introgression (Rollins, 1949).
The Genus Crepis. This dandelion relative has been studied more
thoroughly than any other genus of its size from the point of view of
experimental taxonomy. Ε. B. Babcock and his students worked for 30
years at the University of California to gain as complete a picture as
possible of the origin, development, and speciation in this genus of nearly
200 species. As a geneticist, Babcock undertook the study to help make
taxonomy a more dynamic part of biology by broadening the approach
to its problems. His was the approach of the experimental taxonomist—
to synthesize evidence from all available sources that might throw light
on the relationships between species and larger groups of organisms.
His ultimate goal, a systematic treatment based on all available evidence,
was realized with the publication of his classic monograph (Babcock,
1947a, b).
Babcock has pointed out that the criteria of phylogenetic relationships
in Crepis represent five different disciplines: the first in importance for
classifying all the species into 27 sections and arranging these in an approximate phyletic series is comparative morphology; the second is
chromosome numbers of the species; the third is cytogenetic analysis of
certain interspecific hybrids, which confirmed conclusions reached on
phyletic relationships of the parent species as determined by the first
two criteria and revealed the evolutionary process by which reduction
in chromosome number in the series of species has been accomplished;
the fourth is purely genetic data on interspecific hybrids, which substantiated the groupings of species into sections on a morphological basis
and demonstrated the importance of gene mutation as a cause of speciation; and the fifth is a study of geographic distribution in the genus as
a whole, which, in the light of well-established paleobotanical evidence,
has led to the formulation of a phylogenetic history of the genus (Babcock, 1944b).
Babcock set up a system of classification in Crepis based on probable
phylogenetic sequence—"an orderly progression from the oldest and
most primitive species to the youngest or most advanced species." His
first classification was based on morphology and then checked against
all the other evidence that he could muster. The morphology suggested
93
facultatively apomictic. It is presumed that the plants of higher chromosome numbers in P. incanum reproduce similarly. Introgression is more
prevalent in the polyploid, facultatively apomictic populations than in
those at the 36-chromosome level. It is highly improbable that these two
species evolved in separate areas and have since come together. Rather,
evolution appears to have taken place, including polyploidy and finally
apomicty, under conditions of continuous introgression (Rollins, 1949).
The Genus Crepis. This dandelion relative has been studied more
thoroughly than any other genus of its size from the point of view of
experimental taxonomy. Ε. B. Babcock and his students worked for 30
years at the University of California to gain as complete a picture as
possible of the origin, development, and speciation in this genus of nearly
200 species. As a geneticist, Babcock undertook the study to help make
taxonomy a more dynamic part of biology by broadening the approach
to its problems. His was the approach of the experimental taxonomist—
to synthesize evidence from all available sources that might throw light
on the relationships between species and larger groups of organisms.
His ultimate goal, a systematic treatment based on all available evidence,
was realized with the publication of his classic monograph (Babcock,
1947a, b).
Babcock has pointed out that the criteria of phylogenetic relationships
in Crepis represent five different disciplines: the first in importance for
classifying all the species into 27 sections and arranging these in an approximate phyletic series is comparative morphology; the second is
chromosome numbers of the species; the third is cytogenetic analysis of
certain interspecific hybrids, which confirmed conclusions reached on
phyletic relationships of the parent species as determined by the first
two criteria and revealed the evolutionary process by which reduction
in chromosome number in the series of species has been accomplished;
the fourth is purely genetic data on interspecific hybrids, which substantiated the groupings of species into sections on a morphological basis
and demonstrated the importance of gene mutation as a cause of speciation; and the fifth is a study of geographic distribution in the genus as
a whole, which, in the light of well-established paleobotanical evidence,
has led to the formulation of a phylogenetic history of the genus (Babcock, 1944b).
Babcock set up a system of classification in Crepis based on probable
phylogenetic sequence—"an orderly progression from the oldest and
most primitive species to the youngest or most advanced species." His
first classification was based on morphology and then checked against
all the other evidence that he could muster. The morphology suggested